GammaRay Bursts

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Gamma-Ray Bursts The Brightest Explosions Since
the Big Bang
H. A. Bethe Lecture March 6, 2002
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Electromagnetic radiation of which ordinary
visible light is an example can be
characterized by its wavelength. Gamma-rays
have the shortest wavelengths of all and are very
penetrating. They pass easily through a block of
wood but not through the Earths
atmosphere. Consequently cosmic sources of
gamma-rays can only be seen by satellites,
rockets, and high flying balloons that go above
most of the Earths atmosphere.
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Nuclear Test Ban Treaty, 1963 First Vela
satellite pair launched 1963
Velar to watch
The Vela 5 satellites were placed in orbit by
the Advanced Research Projects of the DoD and
the AEC. Launched on May 23, 1969 into high
earth orbit (118,000 km), this pair of
satellites and their predecessors, Vela 4,
discovered the first gamma-ray bursts. The
discovery was announced by Klebesadel, Strong,
and Olson (ApJ, 182, 85) in 1973.
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First Gamma-Ray Burst
The Vela 5 satellites functioned from July, 1969
to April, 1979 and detected a total of 73
gamma-ray bursts in the energy range 150 750
keV (n.b, radiation is sometimes defined by
its energy measure in electron volts. 0.3 to 30
keV approximately is x-rays. Greater than 30 keV
is gamma-rays)
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Typical durations are 20 seconds but there
is wide variation both in time- structure and
duration. Some last only hundredths of a
second. Others last thousands of seconds.
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A Cosmic Gamma-Ray Burst, GRB for short, is a
brief, bright flash of gamma-rays lasting
typically about 20 seconds that comes from an
unpredictable location in the sky. Some, in
gamma-rays, are as bright as the planet Venus.
Most are as bright as the visible stars. It is
only because of the Earths atmosphere and the
fact that our eyes are not sensitive to
gamma-rays that keeps us from seeing them
frequently. With appropriate
instrumentation, we see about one of these per
day at the Earth. They seem never to repeat
from the same source.
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A problem in 1973 was and still is that
gamma-ray detectors(CsI and NaI) do not give
much information on directect.
Yes, there are gamma-raysbut I dont know where
they came from.....
but I do know when ...
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One can get information on directionality from
timing if there are more than one detectors.
BANG !
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2
If 1 hears the sound later than 2 the sound
is somewhere to the right.
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• Interstellar warfare
• Primordial black hole evaporation
• Flares on nearby stars
• Distant supernovae
• Neutron star quakes
• Comets falling on neutron stars
• Comet anti-comet annihilation
• Thermonuclear explosions on neutron stars
• Name your own ....

uncertainty in distance a factor of one
billion.
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In 1993 there were 135 models
nb. 27, 42, 105, 126
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Most of them involved neutron stars in our own
Galaxy (quakes, comets falling, thermonuclear
runaways, etc.) The expected distribution on the
sky if Galactic neutron stars were responsible
should center around the Galactic equator.
Plane of the MilkyWay Galaxy
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So basically we wandered in the wilderness for
twenty years ... (1973 1993).
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Compton Gamma-Ray Observatory
April 5, 1991 June 4, 2000
BATSE Module
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Observed
Expected if havent reached any edge yet
log number of sources
log sensitivity
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This posed a problem for the models that had GRBs
in our own Galaxy ..


















X









We are not at the center of our galaxy so we
should see more bursts towards the center than
in the opposite direction,









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• Isotropy could mean three things
• Very nearby bursts centered on the Earth e.g.,
• the Oort cloud of comets
• A very extended spherical halo around the Galaxy
  much bigger than the distance from here to
  the center of the Galaxy
• Bursts very, very far away billions of light
  years

What we really needed was source identifications.
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High Energy Transient Explorer (HETE-1)
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HETE-1 conceived July, 1981, Santa Cruz born
Nov 4, 1996 died Nov 5, 1996
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BeppoSax (1996-2002)
(Italian-Dutch X-ray astronomy mission)
MEC S and LECS (medium and lowenergy x-ray
sensors, 1 arc min positions)
(2-30 keV 20x20 degree FOV angular resolution
5 arc min)
The scintillator anti-coincidence shields of the
Phoswich detector are able to detect gamma-rays
60-600 keV and get crude angular information
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BeppoSax GRB 970228 (discovered with WFC)
Feb 28, 1997 (8 hr after GRB using MECS)
March 3, 1997 (fainter by 20)
Each square is about 6 arc min or 1/5 the moons
diameter
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GRB 970228
William Hershel Telescope
Isaac Newton Telescope
Groot, Galama, von Paradijs, et al IAUC 6584,
March 12, 1997
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Wagner, Foltz, and Hewet IAUC 6581 using the
MMT get a crude spectrum of the host galaxy.
Estimate z 0.5 March 10, 1997
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Later ....
Spectrum of the host galaxy of GRB 970228
obtained at the Keck 2 Telescope. Prominent
emission lines of oxygen and neon are indicated
and show that the galaxy is located at a
redshift of z 0.695. (Bloom, Djorgovski, and
Kulkarni (2001), ApJ, 554, 678. See also GCN
289, May 3, 1999.
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From the red shift a distance can be inferred
billions of light years. Far, far outside our
galaxy. From the distance and brightness an
energy can be inferred.
1.6 x 1052 erg in gamma rays alone
This is 13 times as much energy as the sun will
radiate in its ten billion year lifetime, but
emitted in gamma-rays in less than a minute. It
is 2000 times as much as a really bright
supernova radiates in several months.
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GRB 990123
This shows the light curve as seen by BATSE
starting at 94656 UT on January 23, 1999. The
burst was seen simultaneously by the WFC on
BeppoSax. The position was promptly determined to
a few degrees by BATSE and shortly thereafter to
5 arc min by BeppoSax.
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The Robotic Optical Transient Search Telescope
(ROTSE-1) by Akerlof et al, located at Los
Alamos, images a piece of the sky 16.5 degrees
across. It is able to slew anywhere in the sky in
about 10 s. It was notified of GRB 990123 within
4 s of the onset and was taking data 22 s into
the event. The first three ROTSE points are shown
superimposed on the BATSE light curve.
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m 8.95 47 s, 5 s exposure
m 11.82 22 s, 5 s exposure
m 10.08 72 s, 5 s exposure
m 13.22 259 s, 75 s exposure
m 14.0 447 s, 75 s exposure
m 14.53 612 s, 75 s exposure
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And Palomar finds a light where no light was
before. This ID was made 3 hours after the burst
following a 5AM wake-up call to Palomar from
Italy
GRB 990123
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BeppoSax observations of GRB 990123 6 to 33
hours after the burst and 34 to 64 hours after
the burst. The x-ray source is clearly fading.
Each grid box is 12 x 15 arc min. The error
in position is then about 1.5 arc min.
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Two HST images of GRB 990123. The image on the
left was taken February 8, 1999, the one on the
right March 23, 1999. Each picture is 3.2 arc
seconds on a side. Three orbits of HST time were
used for the first picture two for the second
hence the somewhat reduced exposure.
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The spectrum of host galaxy (Kelson et al, IAUC
7096) taken using the Keck Telescopes gives a
redshift of 1.61. Given the known brightness of
the burst (in gamma-rays) this distance implies
an energy of over several times 1054 erg. About
the mass of the sun turned into pure energy.
Had this burst occurred on the far side of our
Galaxy, at a distance of 60,000 light years, it
would have been as bright in gamma-rays as
the sun. This is ten billion times brighter than
a supernova and equivalent to seeing a one
hundred million trillion trillion megaton
explosion.
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As of February, 2002, 129 GRBs had been
promptly localized by various satellites
(starting in 1997). X-ray afterglows had been
detected from 40 and optical afterglows from 28.
Red shifts (distances) were determined for
about 21. Typical values are z 1. The largest is
3.42 (GRB 971214) corresponding to emission when
the universe was about 15 its present age.
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The typical energy is 1053 erg or about 5 of the
mass of the sun turned to pure energy according
to E mc2
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But are the energies required really that great?
Earth
If the energy were beamed to 0.1 of the sky,
then the total energy could be 1000 times less
Earth
Nothing seen down here
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But then there would be a lot of bursts that we
do not see for every one that we do see. About
1000 in fact.
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Earth
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Quasar 3C 175 as seen in the radio
Quasar 3C273 as seen by the Chandra x-ray
Observatory
Artists conception of SS433 based on
observations
Microquasar GPS 1915 in our own Galaxy time
sequence
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It also turns out that in order to get the
spectrum and time scales for GRBs correct the
beam must be relativistic, that is it must
travel at very close to the speed of
light.Otherwise the bursts would be much softer
and last much longer.
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It is a property of matter moving close to the
speed of light that it emits its radiation in a
small angle along its direction of motion. The
angle is inversely proportional to the Lorentz
factor
This offers a way of measuring the beaming angle.
As the beam runs into interstellar matter it
slows down.
Measurements give an opening angle of about 5
degrees.
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Frail et al. Nature, (2001), for 17 GRBs with
known redshifts and afterglow light curves.
Using the angles inferred from this sort of
analysis of the afterglows, Frail et al find
that an inverse correlation exists between the
apparent energy of the burst and this
angle. Correcting for the fact that the burst is
beamed to a small part of the sky, they find a
typical energy in gamma-rays is 5 x 1050 erg.
For a reasonable conversion efficiency between
explosion energy and gamma-rays (20), the total
jet energy is about 2 x 1051, not so different
from an ordinary supernova.
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Requirements on the Central Engine and its
Immediate Surroundings

• Provide adequate energy to material moving close
  to the speed of light (2 x 1051 erg)
• Collimate the emergent beam to approximately 5
  degrees
• Last approximately 10 s
• Make bursts in star forming regions

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Merging Neutron Stars
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Merging neutron star - black hole pairs
Strengths a) Known event b)
Plenty of angular momentum c)
Rapid time scale d) High
energy e) Well developed
numerical models
Weaknesses a) Outside star forming regions
b) Beaming and energy may be
inadequate for long
bursts
But this model may still be good for a class of
bursts called the short hard bursts for which
we have no counterpart information yet.
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The Collapsar Model (aka the Hypernova)
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Usually massive stars make supernovae. Their iron
core collapses to a neutron star and the energy
released explodes the rest of the star. But what
if the explosion fizzled? What if the iron
core collapsed to an object too massive to be a
neutron star a black hole. A star without
rotation would then simply disappear.... But
what if the star had too much rotation to all
go down the (tiny) black hole? If supernovae are
the observational signal that a neutron star has
been born, what is the event that signals the
birth of a black hole?
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In the vicinity of the rotational axis of the
black hole, by a variety of possible processes,
energy is deposited.
The exact mechanism for extracting this energy
either from the disk or the rotation of the
black hole is fascinating physics, but is not
crucial to the outcome, so long as the energy is
not contaminated by too much matter.
7.6 s after core collapse high viscosity case.
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SN 1998bw/GRB 980425
NTT image (May 1, 1998) of SN 1998bw in the
barred spiral galaxy ESO 184-G82Galama et al,
AA S, 138, 465, (1999)
WFC error box (8') for GRB 980425 and two NFI
x-ray sources. The IPN error arc is also shown.
1) Were the two events the same thing? 2) Was
GRB 980425 an "ordinary" GRB seen off-axis?
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A 1053 erg event situated 30,000 light years
away (distance from here to the Galactic center)
would give as much energy to the earth in 10
seconds as the sun equivalent to a 200 megaton
explosion. Does it matter having an extra sun in
the sky for 10 seconds? Probably not. This is
spread all over the surface of the earth and the
heat capacity of the Earths atmosphere is very
high. Gamma-rays would deposit their energy
about 30 km up. Some bad nitrogen chemistry
would happen. Noticeable yes, deadly to all
living things No.
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Biological Hazards of Gamma-Ray Bursts
Distance Events
Megatons Results (kpc)
/10 by

• 100 1000 200
  Some ozone damage, EMP
  acid
  rain
• 1 1 10
  20,000 Ozone gone, acid rain,
  blindness
  2nd and 3rd degree
  burns
• 0.1 0.01 0.1 two
  million Shock waves, flash incineration,
  
  tidal waves, radioactivity (14C)
• 
  End of life as we know it.

Depends on uncertain efficiency for conversion
of energetic electrons to optical light
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Milagrito detection of GRB 970417a
Atkins et al. (2000), Astrophysical Journal
Letters, 533, L119
Large watertight detector near Los
Alamos. Threshold about 100 GeV (gamma-rays
with 100,000 times more energy and
shorter wavelength than ordinary GRB energies)
Fluence of this burst in BATSE was1.5 x 10-7 erg
cm-2.
100 times more energy in TeV radiation??
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Gamma-Ray Large Area Space Telescope (GLAST)
Sensitive to GRBs in the energy range 20 MeV to
300 GeV Scheduled for launch by NASA 2005
detector follows the paths of electron-positron
pairs created in foils.
Will the high energy emission of GRBs be their
dominant emission?
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HETE-2
October 6, 2000
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Last night ...